It's because the two highest grossing films of all time Avatar and Titanic were both released at the very end of the year where there is zero competition.

I think Star Wars VII could come out the day Nuclear War starts between earth and Mars and still be one of the highest grossing films of all time.

But, to tell the truth I actually like the Christmas release thing. December and the build up to Christmas is one of the few times of the year I'm not totally miserable. So a new Star Wars coming out at the same time may actually make me...happy...